The field of the present invention relates to electrostatically-driven solvent ejection or particle formation. In particular, apparatus, methods, and reduced-conductivity fluid compositions are disclosed herein for electrostatically-driven (ESD) solvent ejection (e.g., spraying or atomization) or particle formation (e.g., formation of particles or fibers, including nanoparticles or nanofibers).
Nanostatics Corporation and GABAE Industries Corporation are parties to a joint research agreement that was in effect before the date the invention claimed herein was made. The invention claimed herein was made on behalf of Nanostatics Corporation as a result of activities undertaken within the scope of the joint research agreement.
The subject matter disclosed herein may be related to subject matter disclosed in co-owned: (i) U.S. non-provisional App. No. 11/634,012 entitled “Electrospraying/electrospinning array utilizing a replacement array of individual tip flow restriction” filed Dec. 5, 2006 (now Pat. No. 7,629,030); (ii) U.S. provisional App. No. 61/161,498 entitled “Electrospinning Cationic Polymers and Method” filed Mar. 19, 2009; (iii) U.S. provisional App. No. 61/256,873 entitled “Electrospinning with reduced current or using fluid of reduced conductivity” filed Oct. 30, 2009; and (iv) U.S. non-provisional App. No. 12/728,070 entitled “Fluid formulations for electric-field-driven spinning of fibers” filed Mar. 19, 2010 (now Pat. No. 8,518,319). Each of said provisional and non-provisional applications is hereby incorporated by reference as if fully set forth herein. Each of said applications was made on behalf of, and is owned by, Nanostatics Corporation.
“Electrospinning” and “electrospraying” conventionally refer to the production of, respectively, fibers or droplets, which may be “spun” as fibers or “sprayed” as droplets by applying high electrostatic fields to one or more fluid-filled spraying or spinning tips (i.e., emitters or spinnerets). Under suitable conditions and with suitable fluids, so-called nanofibers or nanodroplets can be formed from a Taylor cone that forms at each tip (although the terms are also applied to production of larger droplets or fibers). The high electrostatic field typically (at least when using a conventional, relatively conductive fluid) produces the Taylor cone at each tip opening from which fibers or droplets are emitted, the cone having a characteristic full angle of about 98.6°. The sprayed droplets or spun fibers are typically collected on a target substrate typically positioned several tens of centimeters away; solvent evaporation from the droplets or fibers during transit to the target typically plays a significant role in the formation of the droplets or fibers by conventional electrospinning and electrospraying. A high voltage supply provides an electrostatic potential difference (and hence the electrostatic field) between the spinning tip (usually at high voltage, either positive or negative) and the target substrate (usually grounded). A number of reviews of electrospinning have been published, including (i) Huang et al, “A review on polymer nanofibers by electrospinning and their applications in nanocomposites,” Composites Science and Technology, Vol. 63, pp. 2223-2253 (2003), (ii) Li et al, “Electrospinning of nanofibers: reinventing the wheel?”, Advanced Materials, Vol. 16, pp. 1151-1170 (2004), (iii) Subbiath et al, “Electrospinning of nanofibers,” Journal of Applied Polymer Science, Vol. 96, pp. 557-569 (2005), and (iv) Bailey, Electrostatic Spraying of Liquids (John Wiley & Sons, New York, 1988). Details of conventional electrospinning materials and methods can be found in the preceding references and various other works cited therein, and need not be repeated here.
Conventional fluids for electrospinning (melts, solutions, colloids, suspensions, or mixtures, including many listed in the preceding references) typically possess significant fluid conductivity (e.g., ionic conductivity in a polar solvent, or a conducting polymer). Fluids conventionally deemed suitable for electrospinning have conductivity typically between 100 μS/cm and about 1 S/cm (Filatov et al; Electrospinning of Micro-and Nanofibers; Begell House, Inc; New York; 2007; p 6). It has been observed that electrospinning of nanometer-scale fibers using conventional fluids typically requires conductivity of about 1 mS/cm or more; lower conductivity typically yields micron-scale fibers. In addition, conventional methods of electrospinning typically include a syringe pump or other driver/controller of the flow of fluid to the spinning tip or emitter, and a conduction path between one pole of the high voltage supply (typically the high voltage pole) and the fluid to be spun. Such arrangements are shown, for example, in U.S. Pat. Pub. No. 2005/0224998 (hereafter, the '998 publication), which is incorporated by reference as if fully set forth herein. In FIG. 1 of the '998 publication is shown an electrospinning arrangement in which high voltage is applied directly to a conductive emitter (e.g., a spinning tip or nozzle), thereby establishing a conduction path between the high voltage supply and the fluid being spun. In FIGS. 2, 5, 6A, and 6B of the '998 publication are shown various electrospinning arrangements in which an electrode is placed within a chamber containing the fluid to be spun, thereby establishing a conduction path between one pole of the high voltage supply and the fluid. The chamber communicates with a plurality of spinning tips. In any of those arrangements, significant current (typically greater than 0.3 μA per spinning tip, often greater than 1 μA/tip) flows along with the spun polymer material. Conventional electrospinning fluids are deposited on metal target substrates so that current carried by the deposited material can flow out of the substrate (either to a common ground or back to the other pole of the high voltage supply), thereby “completing the circuit” and avoiding charge buildup on the target substrate. Even so, flow rates for electrospinning of conventional fluids are typically limited to a few μL/min/nozzle, particularly if nanofibers are desired (increasing the flow rate tends to increase the average diameter of fibers spun from conventional electrospinning fluids). Electrospinning onto nonconductive or insulating substrates has proven problematic due to charge buildup on the insulating substrate that eventually suppresses the electrospinning process. Application of electric fields greater than a few kV/cm to conventional fluids or to metal spinning tips often leads to arcing between the tip and the target substrate, typically precluding useful electrospinning.
The embodiments shown in the Figures are exemplary, and should not be construed as limiting the scope of the present disclosure or appended claims.